This was the conceit that James Blish used in his fantastic "Cities in Flight" tetralogy, which I highly recommend.

<nods> A very good read, is that. Oh for something like the spindizzy!

Amen to that!

But did you read the omnibus edition that had the astonishing afterword by Richard D. Mullen, which included the chart of history in Spenglerian terms plus Blish's future history?

It was that brief essay that sparked my interest in civilizational studies, eventually bringing me to one of the three greatest books I've ever read in a lifetime of library-scouring: Carroll Quigley's The Evolution of Civilizations.

Self-sustaining offworld colonies may be a necessary step for preserving the species until immortality drugs or The Singularity happens.

Yeah, I too think colonies are a good idea. But let's not start like the explorers of the old age and just claim land that is already inhabited and kill entire civilisations like the Spanish Conquistadors did it with the Inka and Aztek Empires...
But fleeing from Earth and abandoning it because it's just turning into a wasteland like in "Wall-E" is just not a solution, that's what I wanted to say. Take care of our planet so we have a planet to send new colonists into space. Because space IS dangerous, and you don't want to end up like in a sci-fi novel where the fled from earth with 5 colony ships and 4 of them got destroyed...

Regarding immortality: I have read a book some while ago where they found something like a vaccination that made you immortal and it was just given to everyone (well, in the books it was given to everyone for free in Europe. In America you would have to pay exorbitant fees to get the shot). But you weren't allowed to have kids. If they found out you were pregnant, they would send a special agent (kids from mothers that had to give them away because it's not allowed to have kids, and these kids were tortured and used in wicked experiments and made into special soldiers) to find you and give you an anti-vaccination that took your immortality again. And every person that showed signs of age was treated like an outcast.
(Damn, why do I not remember the name of this book?